Security cards are widely used for various applications such as identification purposes (ID cards) and financial transfers (credit cards). Such cards typically consist of a laminated structure consisting of various paper or plastic laminates and layers wherein some of them may carry alphanumeric data and a picture of the card holder. So called ‘smart cards’ can also store digital information by including an electronic chip in the card body.
A principal objective of such security cards is that they cannot be easily modified or reproduced in such a way that the modification or reproduction is difficult to distinguish from the original.
Two techniques frequently used for preparing security documents are laser marking and laser engraving. In literature, laser engraving is often incorrectly used for laser marking. In laser marking, a colour change is observed by local heating of material, while in laser engraving material is removed by laser ablation.
US 2005001419 (DIGIMARK) discloses a colour laser engraving method and a security document including an opaque surface layer and one or more coloured sub-layers. A laser provides openings in the surface layer to expose the colour of the sub-layer thereby creating colour images and text.
WO 2009/140083 (3M) discloses methods to generate a colour image in a multilayer article containing at least one thermally activatable layer coated from a composition including a non-linear light to heat converter, a leuco dye, a thermal acid generator and a solvent. A colour image is formed in the colour forming layer upon activation with non-linear light beam radiation (300-1500 nm).
U.S. Pat. No. 7,158,145 (ORGA SYSTEMS) discloses a three-wavelength system (440, 532 and 660 nm) for applying coloured information to a document by means of wavelength-selective bleaching of chromophoric particles in a layer close to the surface.
U.S. Pat. No. 4,720,449 (POLAROID) discloses a thermal imaging method for producing colour images on a support carrying at least one layer of a colourless compound, such as di- or triarylmethane, by direct application of heat or by conversion of electromagnetic radiation into heat. The laser beam may have different wavelengths, typically in a range above 700 nm with at least about 60 nm apart so that each imaging layer may be exposed separately to convert a colourless triarylmethane compound into a coloured form, such as yellow, magenta, cyan or black, by controlling the focusing depth of the laser beam source to each colour forming layer. The colour forming compositions include di- or triarylmethane compounds, infrared absorbers, acidic substances and binders.
U.S. Pat. No. 4,663,518 (POLAROID) discloses a laser printing method for activating heat sensitive image forming dyes in three different layers on a support to card holder, coloured text and machine readable digital code.
Suitable binders that may be used in U.S. Pat. No. 4,720,449 (POLAROID) and U.S. Pat. No. 4,663,518 (POLAROID) include polyvinyl alcohol, polyvinyl pyrrolidone, methyl cellulose, cellulose acetate butyrate, copolymers of styrene and butadiene, polymethyl methacrylate, copolymers of methyl and ethyl acrylate, polyvinyl acetate, polyvinyl chloride and polyvinyl butyral. These binders are commonly employed in heat sensitive recording elements because of their inertness, i.e. they do not have any adverse effect on the colour formation reaction. However, adhesion problems have frequently been observed with these binders. Adhesion is an important aspect for security documents since poor adhesion allows delamination and thus falsification of the laser marked image inside the security document.
Therefore, it would be desirable to have a secure colour laser marking system for producing security documents which combines good adhesion and good image quality.